Despite tremendous advances in cardiovascular research and clinical therapy, heart disease remains the leading cause of morbidity and mortality in western society and is growing in developing countries. Cardiovascular diseases (CVD) have been subjected to extensive study, covering all major pathological conditions: including ischemic heart disease (IHD) and heart failure (HF) the two primary causes of CVD. Heart failure is characterized by reduced blood supply to the heart muscle with resulting decreased function. It is also appreciated that treating heart failure patients with drugs (such as cardiac glycosides) augment pump function by increasing the contractility of cardiac myocytes and can improve hemodynamics and exercise tolerance. These observations led to the “hemodynamic hypothesis” that heart failure is primarily caused by defective cardiac myocyte contractility. It is important to point out that other factors—changes in cardiac structure (dilation), cell death (apoptosis), altered vascular structure and reactivity, abnormal energy utilization, and neurohormonal disturbances also contribute to the progression of CVD. Cardiac contractile function is, in part, regulated by post-translational modifications (PTMs) to the myofilament. This machinery is directly responsible for the force-generating process. Both dynamic and irreversible PTMs, like phosphorylation, occur to myofilament proteins and have been observed in numerous models of heart disease.
Importantly, IHD can occur acutely, resulting in myocardial stunning or myocardial infarction (heart attack) or chronically which is one common cause of heart failure. Interestingly, in the acute situation, intermittent ischemia events can be protective against a subsequently more severe ischemic event reducing cell death and injury to the heart. This is termed myocardial preconditioning and is known to occur to many other organs, including kidney and skeletal muscle. Preconditioning, in part reduces the drop in cellular pH and increase in calcium concentration that occurs with reperfusion. This condition can effect the peptidyl arginine deiminases (PADs) which are a family of calcium dependent enzymes that post-translationally convert arginine residues on substrate proteins to the non-standard amino acid citrulline.
The enzymatic conversion of arginine into citrulline occurs in physiological processes such as epidermal differentiation, formation of the hair follicle and differentiation of the myelin sheath during development of the central nervous system. It was first linked to human pathology by the demonstration of citrullinated proteins in the synovium of patients with rheumatoid arthritis. More recently, protein citrullination has been described in non-rheumatoid inflammatory synovitis and also in autoimmune neurodegenerative diseases such as multiple sclerosis and Alzheimer's disease. In light of these observations, we asked whether citrullination occurs in the heart and whether this modification will provide insights into the pathologies of specific disease states in cardiac. Furthermore, there have been no investigations to determine if PADs are present in the heart during health or disease events.
The incident rates of heart failure and diastolic dysfunction are increased in rheumatoid arthritis (RA) patients compared to non-RA controls, suggesting that myocardial remodeling occurs as part of the RA disease process. The phenotype of heart failure in RA differs from that of non-RA patients, characterized by fewer symptoms, lower blood pressure, and higher ejection fraction at presentation, suggesting that the pathophysiologic mechanisms underlying the progression to heart failure in RA patients may be different from those of the general population. Recently, it was reported that an association of higher concentration of serum anti-CCP antibodies with lower myocardial mass and smaller left ventricular chamber volume in RA patients without known cardiovascular disease; raising the possibility that RA-specific autoimmunity against citrullinated proteins might mediate changes to myocardial morphology that, in turn, may affect myocardial function. Citrullination, the post-translational modification of basic amino acid arginine to neutral amino acid citrulline results in basic charge loss which can influence the overall charge distribution, isoelectric point, as well as the ionic and hydrogen bond formation. This PTM is crucial as it can alter the physical and chemical properties of proteins, regulating protein folding, distribution, stability, activity and function. The reaction is catalyzed by a set of peptidyl arginine deiminase enzymes (PADs), that are abundant in the rheumatoid synovium but not restricted to RA.
In North America, infectious-cardiomyopathy can occur during or follow viral infections (e.g. coxsackievirus B3, adenoviruses or parvovirus B19). In fact, immunocardiomyopathy is an important cause of HF or sudden death especially in children and young adults. Therefore, we believe that myocardial citrullination would be more abundant in RA compared to other conditions, and that myocardial regions demonstrating citrullination would co-localize with evidence of tissue damage (i.e. myocarditis, fibrosis, etc.) and PADs.